New feature to support unity vertex layout requirement (#2493)

* Update mesh.rs

adjust Vertex layout

* add unity feature

* add unity feature

* document the `unity` feature flag

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui/Cargo.toml

@@ -54,6 +54,10 @@ persistence = ["serde", "epaint/serde", "ron"]
 ## Allow serialization using [`serde`](https://docs.rs/serde).
 serde = ["dep:serde", "epaint/serde", "accesskit?/serde"]
 
+## Change Vertex layout to be compatible with unity
+unity = ["epaint/unity"]
+
+
 [dependencies]
 epaint = { version = "0.20.0", path = "../epaint", default-features = false }
 

crates/epaint/Cargo.toml

@@ -63,6 +63,9 @@ mint = ["emath/mint"]
 ## Allow serialization using [`serde`](https://docs.rs/serde).
 serde = ["dep:serde", "ahash/serde", "emath/serde", "ecolor/serde"]
 
+## Change Vertex layout to be compatible with unity
+unity = []
+
 [dependencies]
 emath = { version = "0.20.0", path = "../emath" }
 ecolor = { version = "0.20.0", path = "../ecolor" }

crates/epaint/src/mesh.rs

@@ -6,6 +6,7 @@ use emath::*;
 /// Should be friendly to send to GPU as is.
 #[repr(C)]
 #[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
+#[cfg(not(feature = "unity"))]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 #[cfg_attr(feature = "bytemuck", derive(bytemuck::Pod, bytemuck::Zeroable))]
 pub struct Vertex {
@@ -22,6 +23,25 @@ pub struct Vertex {
     pub color: Color32, // 32 bit
 }
 
+#[repr(C)]
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
+#[cfg(feature = "unity")]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
+#[cfg_attr(feature = "bytemuck", derive(bytemuck::Pod, bytemuck::Zeroable))]
+pub struct Vertex {
+    /// Logical pixel coordinates (points).
+    /// (0,0) is the top left corner of the screen.
+    pub pos: Pos2, // 64 bit
+
+    /// sRGBA with premultiplied alpha
+    pub color: Color32, // 32 bit
+
+    /// Normalized texture coordinates.
+    /// (0, 0) is the top left corner of the texture.
+    /// (1, 1) is the bottom right corner of the texture.
+    pub uv: Pos2, // 64 bit
+}
+
 /// Textured triangles in two dimensions.
 #[derive(Clone, Debug, Default, PartialEq, Eq)]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]